Performance of advanced integrated circuit (IC) technology is becoming dominated by interconnect RC propagation delays making the introduction of lower capacitance insulators very attractive. The use of low dielectric constant (low-k) materials will be a key challenge for future interconnect technologies. In the case of BiCMOS technology for RF applications, an additional consideration is to minimize parasitic capacitance ofpassive components such as inductors, buses, and bond pads. The use ofhydrogen silsesquioxane (HSQ) with a dielectric constant ofabout 3.0 allowed the construction ofhigh quality spiral inductors in a 0.5 pm BiCMOStechnology. In addition to its low-k properties, the HSQ spin-on dielectric was used for planarization ofthree polycrystalline silicon layers and four levels ofmetal interconnects. The HSQ layer was applied in a single coat application in a non-etchback process that achieved excellent planarity with good crack resistance. The stability ofblanket HSQ films was shown using FTIR spectra, film stress, and capacitance data. Immunity ofdevices to hot carrier lifetime degradation was demonstrated. Low resistance and high yield for long metal via chains were obtained by careful integration ofthe via etch, resist strip and metal deposition processes. Thus we demonstrated the integration of HSQ planarization into an advanced BiCMOS process to take advantage of its excellent planarity and its low dielectric constant.

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Mechanical stress problems in integrated circuit devices are becoming more severe as the number of metal interconnect levels increases and new materials such as low-k dielectrics are introduced. We studied dielectric cracking in a four-level Al-Cu interconnect structure that uses hydrogen silsesquioxane (HSQ), a low dielectric constant (low-k) material. The cracks extended down through the passivation layers to the HSQ layer. For the first time we report on passivation dielectric cracks directly related to the level of residual fluorine in a plasma enhanced chemical vapor deposition (PECVD) reactor. It is shown that a silicon nitride pre-coat deposition removes fluorine (F) from the reactor ambient and prevents the dielectric cracks.

This case study describes the difficulties and challenges failure analysts encountered in their nearly year-long investigation into the cause of cracking in a dielectric film. Despite the trend in microelectronics to use ever more costly and sophisticated equipment, this investigation was conducted using only SEM and optical microscope observations coupled with persistent detective work, which eventually uncovered to the cause.